Various integrated circuit chips are currently used to control flyback converters that supply constant current and constant voltage. FIG. 1 (prior art) illustrates an exemplary prior art constant output current flyback converter 10 controlled on the primary side of a transformer 11. Although flyback converter 10 avoids the cost of an opto-coupler typically used in secondary-side controlled converters, flyback converter 10 requires a relatively expensive integrated circuit package. Flyback converter 10 includes an integrated circuit package with six pins to accommodate a conventional peak-current-mode pulse width modulation (PWM) controller integrated circuit (IC) 12. Typically, IC packages with more pins are more expensive than IC packages with fewer pins.
In addition, the discrete external components of flyback converter 10 also contribute to the manufacturing cost. The external components include transformer 11, a voltage divider resistor network 13, a primary switch 14, a primary-side rectifier 15, a secondary-side rectifier 16 and other resistors and capacitors. Transformer 11 has three windings: a primary-side winding Lp, a secondary-side winding Ls, and an auxiliary winding La. Primary switch 14 is an external metal-oxide semiconductor field-effect transistor (MOSFET). A resistor 17 on the secondary side in FIG. 1 represents the resistive loss of the copper windings of transformer 11. Flyback converter 10 also includes a current sense resistor 18, an output capacitor 19, a start-up resistor 20 and a power capacitor 21. The initial start-up energy for controller IC 12 is provided by resistor 20 and capacitor 21. Once flyback converter 10 is stable, auxiliary winding La of transformer 11 powers IC 12 via primary-side rectifier 15.
FIG. 2 (prior art) illustrates another constant output current flyback converter 22 controlled on the primary side of transformer 11. Flyback converter 22 includes a controller IC 23 that is contained in a four-pin integrated circuit package. The IC package of flyback converter 22 is less expensive than the IC package of flyback converter 10 because controller IC 23 requires four as opposed to six pins. Flyback converter 22 still includes voltage divider resistor network 13. Controller IC 23 receives a feedback signal 24 from auxiliary winding La via a voltage divider resistor network 13 and uses feedback signal 24 to control an external NPN bipolar transistor 25. For additional details on a constant output current flyback converter that can be packaged in a four-pin IC package, see U.S. patent application Ser. No. 11/888,599 entitled “Constant Current and Voltage Controller in a Four-Pin Package with Dual-Use Pin,” filed on Jul. 31, 2007, now U.S. Pat. No. 7,522,431, which is incorporated herein by reference.
A less expensive flyback converter is sought that has fewer external components and that is controlled by a controller IC that is contained in an IC package with fewer pins.